In the manufacture of high-bulk tissue products, such as facial tissue, bath tissue, paper towels, and the like, it is common to use one or more through-air dryers for partially drying the web or to bring the tissue web to a final dryness or near-final dryness. Generally speaking, through-air dryers typically include a rotating cylinder having an upper deck that supports a drying fabric which, in turn, supports the web being dried. In particular, heated air is passed through the web in order to dry the web. For example, in one embodiment, heated air is provided by a hood above the drying cylinder. Alternatively, heated air is provided to a center area of the drying cylinder and passed through to the hood.
When incorporated into a papermaking system, through-air dryers offer many and various benefits and advantages. For example, through-air dryers are capable of drying tissue webs without compressing the web. Thus, moisture is removed from the webs without the webs losing a substantial amount of bulk or caliber. In fact, through-air dryers, in some applications, may even serve to increase the bulk of the web. Through-air dryers are also known to contribute to various other important properties and characteristics of the webs.
Through-air dryers, however, are typically much more expensive to manufacture and ship in comparison to other drying devices. For instance, many conventional through-air dryers include a rotating cylindrical deck that is made from a single piece construction. In order to permit air flow, the cylindrical deck is porous. Further, in order to support the significant loads that are exerted on the deck during operation, the cylindrical deck has a substantial thickness. In the past, the decks have been made from expensive materials, such as stainless steel, and have been manufactured using expensive procedures. For instance, in order to make the decks porous, the decks are typically configured to have a honeycomb-like structure that requires a substantial amount of labor intensive and critical welding. In order to support the cylindrical deck and to control air flow through the deck, many through-air dryers also include internal baffles and seals that further increase the cost of the equipment.
Further, since the cylindrical deck is a one-piece construction, the shipping costs for through-air dryers are exorbitant. For example, since the decks cannot be disassembled, specially designed shipping arrangements usually are required.
Recently, demands have been made to increase the capacity and efficiency of through-air dryers. As such, gas flow rates through the dryers have increased. In order to shield the bearings that allow the dryers to rotate from the gas flow path, the bearings have been shifted in position. For instance, referring to FIG. 1, a simplified diagram of a prior art through-air dryer is illustrated. As shown, the through-air dryer includes a cylindrical deck 1 that is supported by a pair of opposing heads 2. The heads 2 are mounted on a rotating shaft 3.
The through-air dryer further includes a pair of bearings 4. The bearings 4 allow for the shaft 3 to rotate. In order to prevent the bearings from being exposed to the hot gas flow traveling through the through-air dryer, the bearings are typically spaced a significant distance from the heads 2. Unfortunately, as a result of the placement of the bearings 4, moments represented by the arrows 5 are created when a load 6 is placed on the through-air dryer during operation. The moments need to be supported by the shaft 3, the heads 2, and the cylindrical deck 1. Thus, due to the presence of the moments, even greater deck thicknesses and massive heads are required in designing the through-air dryer, further increasing the cost to manufacture the dryer and the cost to ship the dryer. An added problem with the existing design is that significant stresses are caused by the differential expansion of components during the heating of the through-air dryer and by the differential temperatures of the through-air dryer during steady-state operation. The safest way to start up a traditional through-air dryer is to limit the warm up rate to a few degrees per minute to allow all parts to equilibrate to the same temperature. This subjects the dryer to lowest differential loads, but there are always stresses induced with a rigid design. Another method to limit the effect of differential expansion from temperature is by the use of exotic materials that have different rates of thermal expansion. For example, the deck, which is typically thin and heats up faster that the support structure, can be made from a material that has a lower coefficient of thermal expansion. This net thermal expansion rate between the deck and support structure is more similar reducing stress. While this helps to alleviate the problem, the cost of the through-air dryer is much higher because of the expense of special materials and the special machining and handling necessary to weld them.
As such, a need currently exists for a through-air dryer design that is simple to produce, controls the loads and moment on the structure, is easy to ship and is not practically limited in size. A need also exists for a through-air dryer design that has a lower capital cost and may be disassembled for facilitating construction and shipping of the dryer. A need also exists for a through-air dryer design that does not create high moments that must be supported by the dryer structure.